Automatic protection switching (APS) is a technique to achieve high availability by using synchronous optical network (SONET) line redundancy. A standard APS system is illustrated in FIG. 1. A switch 100 is connected to peer APS equipment 110 by a plurality of transmission lines. Asynchronous transfer mode (ATM) cells are transmitted via a set of primary transmission lines 120 and 125 called working lines. The working lines includes a SONET line 120 for sending data and control information from the peer APS equipment 110 to the switch 100 and a SONET line 125 for sending data and control information back to the peer APS equipment 110. A second set of transmission lines is the protection set of lines. The protection set of lines also includes SONET lines 130 and 135.
A single framer device 140 connects the protection set of lines and the working set of lines to the ATM interface. The single framer 140 terminates a first SONET Line layer 141 for the working set of lines and a second SONET Line layer 142 for the protection set of lines. The single framer 140 also terminates a first SONET Section layer 143 for the working set of lines and a second SONET Section layer 144 for the protection set of lines. Both the working set of lines from the first line layer 141 and the protection set of lines from the second line layer 142 feed into a single common SONET Path layer 145. The single common path layer 145 then forwards the payload (i.e. ATM cells) through an interface 150, such as the Utopia interface, to the ATM device 160. In the reverse direction, cells originating from ATM device 160 reach the common path layer via an interface 170.
The common SONET Path can generate indications back to the peer APS equipment 110. These indications include remote defect indication (RDI) at the path layer (RDI-P) and far end block errors (FEBE). An RDI-P indication can be caused by loss of signal, loss of frame, loss of pointer, loss of cell delineation, an alarm indication signal (AIS) at the Line layer, an AIS at the Path layer (AIS-P), a signal label mismatch, an unequipped signal indication, and a path trace mismatch defect on the selected line.
As seen from FIG. 1, protection switching takes place between the SONET Line layer and the SONET Path layer, as mandated by existing standards. A selector 180 is positioned between common Path layer and Line layers to allow selection of data from the working line or the protection line for transmission downstream. A bridge 190 is positioned between common Path layers and Line layers to allow transmission of data to the working line or to the protection line or both. In 1+1 APS configuration, the bridge 190 transmits data to both lines. In 1:1 architecture, the bridge 190 transmits data to either the working line or to the protection line.
Under existing APS standards, certain defects on the selected line such as Loss of Signal (LOS), Loss of Frame (LOF), Loss of Pointer (LOP), AIS-P, AIS at the Line layer (AIS-L), Signal Label Mismatch, Unequipped and Loss of Cell Delineation (LOCD) each require RDI-P generation. In 1+1 APS operation, an RDI-P is generated and cleared for the working and protection lines simultaneously. In 1:n APS operation, RDI-P is transmitted on the line on which the bridge is positioned.